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File Locking

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File Locking Programmation Systèmes Cours 8 — IPC: File Locking Stefano Zacchiroli [email protected] Laboratoire PPS, Université Paris Diderot - Paris 7 15 novembre 2011 URL http://upsilon.cc/zack/teaching/1112/progsyst/ Copyright © 2011 Stefano Zacchiroli License Creative Commons Attribution-ShareAlike 3.0 Unported License http://creativecommons.org/licenses/by-sa/3.0/ Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 1 / 44 Outline 1 Synchronization and file locking 2 flock 3 fcntl-based locking Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 2 / 44 Outline 1 Synchronization and file locking 2 flock 3 fcntl-based locking Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 3 / 44 Without synchronization the following might happen: 1 process A: read sequence number (obtains n) 2 process B: read sequence number (obtains n) 3 process A: use sequence number n 4 process A: write n + 1 back to file FAIL. 5 process B: use sequence number n 6 process B: write n + 1 back to file Process synchronization Consider again the need of distributing unique sequential identifiers. We would like to implement it without a central server, relying on process cooperation. we can store the global counter in a shared file each processes can access it as follows: 1 read current sequence number (n) from file 2 use sequence number n 3 write n + 1 back to file Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 4 / 44 Process synchronization Consider again the need of distributing unique sequential identifiers. We would like to implement it without a central server, relying on process cooperation. we can store the global counter in a shared file each processes can access it as follows: 1 read current sequence number (n) from file 2 use sequence number n 3 write n + 1 back to file Without synchronization the following might happen: 1 process A: read sequence number (obtains n) 2 process B: read sequence number (obtains n) 3 process A: use sequence number n 4 process A: write n + 1 back to file FAIL. 5 process B: use sequence number n 6 process B: write n + 1 back to file Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 4 / 44 File locking File locking is one of the simplest ways to perform synchronization among cooperating processes. With file locking, each process: 1 place a lock on the file 2 executes its critical section ñ e.g. read sequence number n, use it, write n + 1 back 3 remove the lock The kernel maintains internal locks associated with files on the filesystem and guarantees that only one process at a time can get a file lock (and therefore be in the critical section). The rationale for associating locks with files is that synchronization is often used in conjunction with file I/O, on shared files. file locks are also used for general process synchronization, given its simplicity and the pervasiveness of filesystem on UNIX Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 5 / 44 Advisory and mandatory locking We speak about mandatory locking when the locking system forbids a process to perform I/O, unless it has obtained a specific file lock. We speak about advisory locking when acquiring locks before proceeding into the critical section is a convention agreed upon by cooperating processes. Traditionally, on UNIX the most common kind of file locking is advisory locking. We will focus on it. Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 6 / 44 File locking APIs There are two main APIs for placing file locks on UNIX: flock, which places locks on entire files fcntl (AKA record locking), which can be used to place locks on regions of files ñ fcntl offers a superset of flock’s features, but it’s also plagued by some design issues. Pick one, do not mix the two! History in early UNIX system there was no support for file locking; that made impossible to build safe database systems on UNIX flock originated on BSD circa 1980. Nowadays, it is often used for general process synchronization. fcntl-based locking descends from System V circa 1984 and is nowadays a popular file locking API. POSIX.1 chose to standardize the fcntl approach Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 7 / 44 Outline 1 Synchronization and file locking 2 flock 3 fcntl-based locking Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 8 / 44 flock #include <sys/file.h> int flock(int fd, int operation); Returns: 0 if OK, -1 on error fd is a file descriptor referencing an open file operation is an OR of the following flags: flag meaning LOCK_SH place a shared lock LOCK_EX place an exclusive lock LOCK_UN unlock instead of locking LOCK_NB make a non-blocking lock request Note: locks are requested on open files. Hence, to lock a file a process should have the permissions needed to open it. Other than that, read/write/exec permissions are irrelevant. Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 9 / 44 flock semantics At any time, a process can hold 0 or 1 locks on an open file. Lock kinds. Two alternative kinds of locks are supported: a shared lock (AKA “read lock”) can be hold by several processes at a time an exclusive lock (AKA “write lock”) can be hold by only one process at a time and also inhibits the presence of shared locks Blocking behavior. If the request lock cannot be granted, the process will be blocked. Unblocking will happen as soon as the lock can be granted, atomically with it. To avoid blocking, the flag LOCK_NB can be used. With it, instead of blocking, flock will fail with errno set to EWOULDBLOCK. Unlock. To release the current held lock, the flag LOCK_UN can be used. Locks are also automatically released upon close. Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 10 / 44 flock — example #include <errno . h> #include <stdio . h> #include <string . h> #include <sys/file .h> #include <unistd . h> #include "apue . h" #define LOCK_PATH "my−lock " int main ( int argc , char ** argv ) { int fd , lock ; i f (argc < 2 || strlen(argv[1]) < 1) { printf("Usage: ./flock ( x | s ) [ n ]\n"); exit (EXIT_FAILURE); } lock = ( argv [ 1 ] [ 0 ] == ’x ’ ) ? LOCK_EX : LOCK_SH; i f (argc >= 3 && strlen(argv[2]) >= 1 && argv[2][0] == ’n’) lock |= LOCK_NB; Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 11 / 44 flock — example (cont.) i f ( ( fd = open (LOCK_PATH, O_RDONLY) ) < 0) err_sys("open error"); i f (flock(fd, lock) < 0) { i f ( errno == EWOULDBLOCK) err_sys("already locked" ); else err_sys("flock error (acquire)"); } printf("lock acquired, sleeping...\n"); sleep ( 8 ) ; i f (flock(fd, LOCK_UN) < 0) err_sys("flock error (release)"); exit (EXIT_SUCCESS); } /* end of flock.c */ Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 11 / 44 flock — example (cont.) Demo Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 11 / 44 flock context Obtained file locks are stored by the kernel in the table of open files they are neither stored in the file descriptor itself nor stored in filesystem i-nodes This choice has important consequences on flock inheritance: upon FD duplication( dup/dup2), locks are inherited by the new file descriptors upon fork, we know that the entry in the table of open files is shared; therefore, locks are preserved as well upon exec, open files are left untouched; once more, locks are preserved Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 12 / 44 there is only one lock, stored in the open file table entry pointed by both fd1 and fd2 it will be released upon LOCK_UN flock inheritance gotchas flock context can result in surprising behavior, if we’re not careful. Here are some common “gotchas”.1 flock ( fd1 , LOCK_EX ) ; fd2 = dup(fd); flock ( fd2 , LOCK_UN ) ; 1as flock is not standard, we show BSD / Linux behavior Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 13 / 44 flock inheritance gotchas flock context can result in surprising behavior, if we’re not careful. Here are some common “gotchas”.1 flock ( fd1 , LOCK_EX ) ; fd2 = dup(fd); flock ( fd2 , LOCK_UN ) ; there is only one lock, stored in the open file table entry pointed by both fd1 and fd2 it will be released upon LOCK_UN 1as flock is not standard, we show BSD / Linux behavior Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 13 / 44 there are two different locks, as separate open create separate entries in the open file table (possibly for the same file) the 2nd flock will block (forever. ) as we are trying to acquire two different exclusive locks on the same file Warning A process can lock himself out of a flock lock, if not careful. flock inheritance gotchas (cont.) fd1 = open("foo.txt" , O_RDWR); fd2 = open("foo.txt" , O_RDWR); flock ( fd1 , LOCK_EX ) ; flock ( fd2 , LOCK_EX ) ; Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 14 / 44 flock inheritance gotchas (cont.) fd1 = open("foo.txt" , O_RDWR); fd2 = open("foo.txt" , O_RDWR); flock ( fd1 , LOCK_EX ) ; flock ( fd2 , LOCK_EX ) ; there are two different locks, as separate open create separate entries in the open file table (possibly for the same file) the 2nd flock will block (forever. ) as we are trying to acquire two different exclusive locks on the same file Warning A process can lock himself out of a flock lock, if not careful. Stefano Zacchiroli (Paris 7) IPC: file locking 15 novembre 2011 14 / 44 there is only one lock, inherited through fork upon LOCK_UN, the child will release the lock for both himself and the parent This is actually useful: it allows to transfer a lock from parent to child without race conditions.
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